The present invention relates to an improved process for the removal of sulfur oxides from sulfur oxide-containing gases, and more particularly, to an improved process for the simultaneous absorption of sulfur dioxide and production of ammonium sulfate from gas containing sulfur dioxide.
Sulfur dioxide emission control from such sources as fossil fuel fired boilers, smelters, sulfuric acid plants, pulp and paper mill operations and the like is required by law in many countries to mitigate the serious environmental and health damage that is associated with sulfur dioxide. The most widely practiced method for sulfur dioxide control is based upon limestone or lime contact with flue gases in the form of aqueous slurry. In most instances, the by-product is either discarded as a land fill or converted into gypsum for use in wall board and cement manufacture. In a few instances, other alkaline reagents, such as sodium compounds, magnesium compounds and ammonia have been used with recovery of useful by-products, such as pure sulfur dioxide, sulfuric acid and sulfur.
Ammonia and ammoniacal scrubbing solutions are also well known for flue gas desulfurization. In U.S. Pat. No. 4,690,807, which is incorporated herein by reference in its entirety, gases containing at least one sulfur oxide are treated with aqueous ammonia, and ammonium sulfate is produced in a single vessel which includes an absorption tower and liquor reservoir. The sulfur oxide gas or gases are removed from gases containing sulfur oxides by contact of the gas with an aqueous solution of ammonium sulfate. As the sulfur oxide gas, such as sulfur dioxide, is absorbed by the ammonium sulfate solution, it becomes acidic. The acidic solution in U.S. Pat. No. 4,690,807 is neutralized by injection of ammonia into the ammonium sulfate solution to maintain a desired pH level sufficient to prevent excessive ammonia loss. An oxidizing medium, such as air, is injected into the neutralized ammonium sulfate solution containing the absorbed sulfur oxide gas, leading to the formation of ammonium sulfate. The ammonium sulfate product can be withdrawn as a suspension of ammonium sulfate crystals, as a saturated solution and/or as a less-than-saturated solution. In U.S. Pat. No. 4,690,807, recovered ammonium sulfate can be further recrystallized and/or dewatered and dried for ease of handling and storage.
It is generally known that in processes dependent on oxidation rate, the higher the oxidation rate, the more economical the process. It has been determined that the rate of oxidation of ammonium sulfite/bisulfite formed by absorption of sulfur dioxide in ammonium sulfate liquor and reaction with ammonia, is dependent upon the concentration of ammonium sulfate in the liquor and that the rate of oxidation decreases with increasing concentration. Although sulfur dioxide removal from flue gas and simultaneous ammonium sulfate production in the system and process of U.S. Pat. No. 4,690,807 is efficient, it is always desirable and advantageous to improve efficiency in the scrubbing of gases. Thus, it would be advantageous to provide a process and apparatus for the removal of sulfur oxides from gases with the simultaneous production of ammonium sulfate wherein the rate of oxidation is increased by decreasing the concentration of the ammonium sulfate used to absorb the sulfur oxide gases.